1. Field of the Invention
This invention relates to implantable techniques for stimulating electrically excitable tissue, and more particularly relates to such techniques for improving the uniformity of the response to tissue stimulation.
2. Description of the Related Art
Stimulation of neural tissue to reduce pain is a known therapy. Conscious sensation from stimulation is called paresthesia. It is commonly believed that pain relief with stimulation is strongly correlated to the overlap of the area of paresthesia and the area of pain.
Frequently, electrodes are implanted adjacent the spinal cord in order to create the electrical stimulation of neural tissue which creates the paresthesia. In some patients with such an implant, the paresthesia changes dramatically when there is a change in body position, such as standing up from a reclining position. It is difficult if not impossible for the user to properly adjust the electrical stimulation to create an essentially constant or uniform paresthesia. Sometimes the patient will use a lower level of stimulation than optimal in order to avoid "shocks".
Currently available totally implantable electrical stimulation systems offer two amplitude levels of stimulation, and employ a magnet to jump between levels depending on perceived paresthesia. U.S. Pat. No. 5,031,618 (Mullett) describes a system to determine body orientation with respect to gravity, and jumps between levels automatically. Portable programmers are available to adjust amplitude or of the electrical stimulation periodically. Radio frequency systems also allow changing of parameters deliberately. None of these systems is capable of maintaining a substantially constant or uniform degree of paresthesia; and hence, none achieves optimal pain reduction.